Learn Hardcore Finnish: Tämä asunto on isompi kuin edellinen. - This apartment is bigger than the previous one.

Finnish normally forms the comparative of adjectives by adding -mpi to the adjective stem, not by using a separate word like enemmän (more).

iso (big) → isompi (bigger)
kaunis (beautiful) → kauniimpi (more beautiful)

Using enemmän iso would sound wrong or at best very learner‑like. Enemmän is used with many verbs or nouns (e.g. enemmän rahaa – more money), but not to form ordinary adjective comparatives when a regular -mpi form exists.

Is there any difference between isompi and suurempi?

Both can mean bigger / larger, and in many contexts they’re interchangeable.

iso → isompi
suuri → suurempi

Nuances (very general tendencies):

iso/isompi is very common and often a bit more colloquial or neutral.
suuri/suurempi can feel slightly more formal or abstract (e.g. suurempi ongelma – a bigger/more serious problem).

For apartments, isompi and suurempi are both fine; isompi is probably more frequent in casual speech.

What exactly does kuin mean here, and how is it different from kun?

In this sentence kuin means than in a comparison:

isompi kuin edellinen = bigger than the previous one

Kun usually means when or sometimes as (in the sense of “while”):

Kun tulin kotiin, satoi. – When I came home, it was raining.

So:

kuin → used after comparatives and superlatives (enemmän kuin, paras kuin, isompi kuin).
kun → used in time clauses (when).

They are pronounced similarly but function very differently.

Can I leave out kuin and still express “than the previous one”?

You can’t just drop kuin from this exact structure, but Finnish has an alternative pattern that doesn’t use kuin: use the other item in the partitive case.

So you can say:

Tämä asunto on isompi kuin edellinen.
Tämä asunto on edellistä isompi.

Both mean “This apartment is bigger than the previous one.”

The second version literally means “This apartment is the previous (one) bigger”, where edellistä is the partitive of edellinen. You may not say Tämä asunto on isompi edellinen without kuin or case marking; that’s incorrect.

Why is edellinen standing alone? Where is the word “one” in Finnish?

Finnish often lets adjectives stand in for a whole noun phrase when the noun is clear from context. Here:

edellinen literally = previous (adjective)
but in context it means “the previous (apartment)” → “the previous one”

This is very common:

Valitse sopiva. – Choose a suitable (one).
Otan punaisen. – I’ll take the red (one).

So edellinen is an adjective used as a noun, exactly like English “the previous (one)”, where one is often not said in speech.

What form / case is edellinen here, and why that form?

Edellinen here is nominative singular.

The full underlying structure is essentially:

Tämä asunto on isompi kuin edellinen asunto (on).

The compared thing (edellinen asunto) is in the same basic form you’d use for a subject or a dictionary headword: nominative singular. The noun asunto is just omitted because it’s obvious.

If you use the alternative pattern Tämä asunto on edellistä isompi, then edellistä is in partitive singular, because that construction requires the compared element in the partitive.

Why is asunto in the nominative and not in some case like the partitive?

In sentences with olla (to be) that describe what something is, the subject and the predicate (complement) are usually in the nominative case:

Tämä asunto (subject, nominative)
on isompi kuin edellinen (predicate phrase headed by isompi, also nominative in form)

You would use partitive with olla in different meanings, for example to express existence or incompleteness:

Tässä talossa on asuntoja. – There are apartments in this building. (asuntoja = partitive plural)

Here we’re just saying what this apartment is like, so nominative is normal.

Where is the second on? Why isn’t it “…kuin edellinen on”?

Finnish often omits the verb in the second part of a comparison when it would just repeat the same verb:

Full form: Tämä asunto on isompi kuin edellinen (on).
Normal spoken/written form: Tämä asunto on isompi kuin edellinen.

Because on would be identical to the first on, it’s usually dropped. You’d only say …kuin edellinen on if you want extra emphasis or in very clear, careful speech.

Does tämä have to match asunto in case? What would change if the case changed?

Yes, demonstratives like tämä agree in case (and number) with the noun they modify.

Nominative: tämä asunto – this apartment
Partitive: tätä asuntoa – this apartment (as object / in partitive role)
Inessive: tässä asunnossa – in this apartment

In your sentence, Tämä asunto is the subject, so both are in nominative. If the case changed because of grammar, both would change together.

Could I say Tuo asunto on isompi kuin edellinen instead of Tämä asunto…?

Yes.

tämä = this (near the speaker)
tuo = that (a bit further away from both speaker and listener)
se = that/it (often something previously mentioned or mentally pointed to)

So:

Tämä asunto on isompi kuin edellinen. – This apartment is bigger than the previous one.
Tuo asunto on isompi kuin edellinen. – That apartment is bigger than the previous one.

The grammar is the same; only the pointing distance changes.

Could I say Tämä asunto on edellistä isompi instead? Is there any difference in meaning?

You can, and it’s fully natural Finnish:

Tämä asunto on isompi kuin edellinen.
Tämä asunto on edellistä isompi.

Both mean “This apartment is bigger than the previous one.”

Nuance:

isompi kuin edellinen is a straightforward kuin-comparison.
edellistä isompi is a bit more compact and perhaps slightly more colloquial-sounding, but still standard.

There’s no real semantic difference; it’s a stylistic choice.

Why isn’t there any word showing grammatical gender, like he / she / it for the apartment?

Finnish doesn’t have grammatical gender at all:

No gender on nouns (asunto is neither masculine nor feminine).
No gendered pronouns: hän covers both he and she, and se often means it (and sometimes he/she in spoken language).

So nothing in Tämä asunto on isompi kuin edellinen marks gender. Both tämä and edellinen are completely gender‑neutral.

Is the word order Tämä asunto on isompi kuin edellinen fixed, or can I move things around?

The given order is the most natural and neutral:

[Tämä asunto] [on] [isompi] [kuin edellinen].

You have some flexibility, especially for emphasis:

Isompi tämä asunto on kuin edellinen. – Very emphatic, poetic or contrastive.
Tämä asunto on kuin edellinen isompi. – This is not normal Finnish and sounds wrong.

The core pattern you should stick to is:
Subject – verb – adjective (comparative) – kuin + comparison target.